1. Field of the Invention
The present invention relates to a method and an apparatus for detecting trouble in an exhaust-gas recirculation (hereinafter referred to as "EGR") system of an internal combustion engine, particularly to a method and an apparatus capable of detecting trouble in an EGR control valve and the clogging of a passage in the EGR system as well as discriminating whether the clogging occurs upstream or downstream of the EGR control valve.
2. Description of the Related Art
To reduce an amount of No.sub.x in an exhaust gas, an EGR system, as is well known, wherein the exhaust gas flowing through an exhaust passage is recirculated to an intake passage of an internal combustion engine via an EGR passage is used. In the EGR system of such a type, an EGR control valve is usually provided in the EGR passage. The amount of EGR gas to be supplied to the intake passage of the internal combustion engine is controlled by the opening degree of the EGR control valve. However, the recirculation of exhaust gas may be continuously interrupted if the EGR control valve is broken or the EGR passage clogs. If such an inconvenience is left as it is, a large amount of NO.sub.x is continuously discharged because no EGR is conducted. In addition, the reduction of the amount of recirculating exhaust gas or the interruption of EGR due to the trouble of the EGR system is seldom found by the driver.
To solve such a problem, apparatuses, for diagnosing the trouble of EGR system, have been proposed wherein the opening degree of EGR control valve is controlled by the magnitude of a negative pressure introduced into a diaphragm chamber in the EGR control valve partitioned by a diaphragm. For example, Japanese Unexamined Patent Publication (Kokai) No. 63-75345 discloses an apparatus for diagnosing the trouble of EGR system of a diaphragm type comprising a pressure detection means provided upstream of EGR control valve and means for detecting the supply of negative pressure to the EGR control valve. According to the trouble detection apparatus disclosed in the above Patent Publication, "trouble" is determined if no flow of exhaust gas is detected by the pressure detection means when a negative pressure is supplied to the EGR control valve. Contrarily, "trouble" is also determined if the flow of exhaust gas is detected when no negative pressure is supplied to the EGR control valve.
According to this apparatus, however, although trouble in the EGR system can be diagnosed, it is impossible to determine whether the trouble is caused by the malfunction of the EGR control valve or by clogging of the EGR passage. Also, when the EGR passage is clogged, it is impossible to determine whether the clogging has occurred upstream or downstream of the EGR control valve. Accordingly, there is a problem in the above-mentioned trouble detection apparatus in that a long time is required identify the trouble in the EGR system, which results in a time-consuming repair operation.